Webhook
A reverse API: instead of you polling for events, the other service calls YOUR URL when something happens.
A webhook is a user-defined HTTP callback: instead of repeatedly polling a service for new events, you register a URL and the service sends an HTTP POST to it the moment something happens (a payment succeeds, a PR opens). It is push, not pull.
Worked example: Stripe fires a webhook to your /webhooks/stripe endpoint on every successful charge, so you update the order instantly instead of polling their API every few seconds. Gotcha: webhooks are delivered at-least-once and can arrive out of order or be spoofed — so verify the signature, make the handler idempotent (dedupe on the event id), and return 200 fast (do slow work asynchronously) or the sender retries and piles up duplicates.